Effective gene therapy will revolutionize the treatment of the hemophilias. Recombinant adeno-associated virus (rAAV) vectors are considered among the most promising viral vectors for hemophilia gene therapy. The non-pathogenic nature of AAV, the ability to transduce mitotic and post-mitotic cells, and the capacity for stable persistence of rAAV/transgene sequences are unique among all viral vectors. A major obstacle in the application of rAAV in gene therapy for hemophilia A (factor VIII deficiency) is the conflict of the limited packaging capacity of rAAV and the large size of the human FVIII gene. The major rate-limiting aspect of this delivery system has always been the small packaging capacity (5kb) of rAAV. Factor VIII with its large cDNA (7.0 Kb) is an excellent model to test a variety of new approaches for AAV-mediated gene transfer. Here we present compelling evidence supporting the use of AAV vectors for the expression human factor VIII gene therapy. We developed several different novel approaches for the expression of functional factor VIII. First, we developed rAAV vectors carrying a truncated version of the full-length FVIII cDNA. Removal of the B-domain sequence of factor VIII (~4.0 Kb) results in a fully functional protein (termed B-domain deleted, BDD FVIII which express therapeutic levels of functional FVIII in vivo. Despite truncation of the FVIII sequence, the use of small (<250 bp) enhancer/promoter elements is still required for AAV packaging. Further truncation of the FVIII sequence and modification of the transcriptional elements are proposed. Second, AAV dimerization can be used to overcome vector-packaging limitations. AAV proviral DNA is characterized by head-to-tail concatamers. Here, the FVIII gene is divided and packaged into two individual AAV vectors. Dimerization dramatically increased by amplifying the conversion of single to double-strand intermediates. Third, a totally novel RNA repair strategy relies on the use of spliceosome-mediated trans-splicing. Here two independent pre-messenger RNA transcripts are spliced together via the native cellular splicing machinery. We present molecular, protein and functional data demonstrating correction of the FVIII knockout mouse phenotype using this method. Fourth, AAV type 2 is the predominant serotype used for gene transfer studies. We propose that alternate AAV serotypes differ in terms of their cellular tropism. We demonstrate that non-type 2 AAV serotypes effect significantly higher levels of factor IX expression and will be used to test factor VIII expression. Each method will be optimized and in AAV vectors for FVIII production and tested in vivo using immunodeficient and FVIII knockout mice and hemophilic A canines.